Control of exhaust gas flow in an internal combustion engine is commonly used to alter the engine's function or modify its performance. Exhaust gas generated by pistons of an engine is released into its exhaust system during an exhaust cycle. This gas flow may be controlled with the use of variously configured valves positioned and operated in the exhaust system of the engine.
Conventional EGC valves include sliding valves, poppet valves, and valve-in-bore systems. Sliding valves typically have a valve plate that slides into and out of the exhaust gas flow path, thereby controlling the flow. This type of valve may provide adequate flow, because the valve plate mechanism is generally not in the flow path when the valve is in an open position. Therefore, the flow path area is maximized. However, sliding valves have several disadvantages. One, the valve plate slides along a sealing surface during its articulation, which potentially causes excessive wear. Two, friction between the valve plate and the sealing surface may be very large at high-pressure differentials. The actuation system must exert a relatively large amount of energy to provide enough force to overcome such friction. Three, the overall system is often bulky because the actuation system pulls the valve plate straight out from its closed position thereby increasing the overall envelope that the valve takes up.
Poppet valves, such as engine exhaust valves or waste-gate valves, have a face-sealing portion of the valve plate, which is moved away from a sealing surface to open the exhaust flow path. Such valves may provide a failsafe mode in the event of actuation system failure. However, in the presence of a high-pressure differential across the valve plate, a relatively high actuation system force is generally required to open the valve or keep the valve closed (depending on the exhaust flow direction when the valve is open). Furthermore, poppet valves often result in excessive flow restriction in the open position given a portion of the valve mechanism is positioned in the flow path when in the open position.
Valve-in-bore systems include wall contact type valves and 90-degree closing type valves. The wall contact type typically has an elliptical valve plate that is placed on a shaft in the bore. The valve plate is then rotated about the shaft axis, so that it eventually begins restricting the flow. In its closed position, the valve plate contacts the wall of the bore. A relatively small clearance between the valve plate and the bore provides a relatively low exhaust flow leakage. However, the contact of the valve plate on the bore requires a very sturdy actuation system. In addition, the valve plate is often prone to sticking in the bore, particularly with temperature fluctuations.
The 90-degree closing type has a valve plate that is round and slightly smaller in diameter than the bore. The valve plate rotates about a centerline of the bore. In its closed position, the valve plate is perpendicular to the exhaust flow and has relatively small annular space as clearance. This configuration reduces the occurrence of valve plate sticking in the bore compared to the wall contact type, but allows additional leakage of the exhaust flow.
EGC valves perform various functions in engine operation, such as lowering exhaust gas emissions, engine warm-up, engine retarding and exhaust gas recirculation (EGR). Engine retarding using exhaust brakes is well known in the art. Exhaust brakes generally include a restrictor valve mounted in the exhaust system. Such valves restrict the exhaust gas flow from exiting the engine, thereby retarding the engine. Exhaust system pressure is raised and the engine is required to expend substantial energy to continue to cycle. EGR valves reduce the formation of nitrogen oxides (NOx) during combustion by redirecting a portion of the exhaust gas to the intake system of the engine. That portion of the exhaust gas is then mixed with incoming fresh air, resulting in a lower combustion temperature. This limits production of NOx. EGC valves may also divert the exhaust gas to other after-treatment processes for further extraction of pollutants prior to releasing the gas into the atmosphere.
Various considerations influence the design of a particular EGC valve depending on the desired function or performance, such as accurate flow control resolution, sufficient flow capacity, ability to operate reliably in the exhaust system environment, leak proof sealing when the valve is closed, and resistance to sticking when opening the valve. It is desirable that EGC valves accurately meter the exhaust gas flow over a wide range of operating conditions. Generally, more accurate control is required at low flow rates than at high flow rates.
Conventional EGC valves typically rely on actuator positioning to control the amount of flow, and exhibit a proportional, or linear response over their entire range of valve-opening positions. Some conventional systems use pneumatic pressure to actuate a pintle valve, and a pressure-balanced diaphragm with an air bleed to maintain valve position. Alternatively, the pintle valve may be solenoid actuated, and have two gaps wherein a first gap is engaged for low flow and a second gap is engaged for high flow. In either case, the response of such valves remains linear and there is no fine control for accurately metering the exhaust gas flow volume.
Some conventional systems include an electronic component for adjusting a poppet valve for controlling gas flow. The electronic component may trigger valve actuation based on sensed engine temperature. Such valves provide some improvement over pneumatic pressure actuated valves, but still lack fine control for accurately metering the exhaust gas flow because the flow is limited to the linear flow characteristic of the poppet valve.
In addition to accurate flow metering problems, the configurations of many pintle valves and poppet valves are inherently restrictive to exhaust flow. As such, flow control resolution (i.e. the rate at which the effective flow area changes) is adversely affected. The shafts of pintle and poppet valves are typically positioned in the central portion of the exhaust flow duct, thereby partially blocking the flow area in the flow duct. As such, the maximum flow area is reduced. Furthermore, a relatively large actuator stroke is required for actuating many types of conventional linear valves.
Various attempts have been made to overcome flow rate problems associated with conventional EGC valve systems. One such attempt provides for a system having a butterfly valve that allows for a relatively high exhaust flow rate with minimal flow restriction. Another attempt provides a system having an eccentric drive to achieve a relatively high lift with a poppet valve, which improves flow capacity. Another system includes a valve with concentric flow orifices to provide for a relatively high flow capacity with a relatively low stroke compared to a conventional linear actuator and valve systems. Although flow rate capacity may be improved, such systems fail to provide for accurate flow control resolution.
In addition to flow rate problems, soot deposits and condensates may accumulate in the clearances between the moving parts of a conventional EGC valve due to the environment to which such valves are subject. Wide temperature fluctuations and residual products of combustion (i.e. soot and condensates) are present in the exhaust gas. These residual products may build-up and cause incomplete sealing when the valve is seated in its closed position. They may also cause conditions that require high breakaway energy to open the valve from its closed position.
Attempts to reduce soot and condensate build-up have been made for some EGC valves. One attempt provides for a pintle valve design that reduces the thermal inertia of the pintle valve head, which reduces the rate of carbon build-up. Another attempt provides for a EGC valve assembly that can be mounted on the exhaust manifold side of the engine. This avoids condensates that may form and deposit in a valve mounted on the cooler intake side. Such systems have achieved partial success in overcoming problems associated with condensates and soot deposits, but fail to also provide for accurate flow control and/or high flow rate capacity.
Another consideration in EGC valve design is the pressure or force required for actuation of the EGC valve, or “break-away force”. The break-away force influences the size of the actuator hardware. This, in turn, can effect system packaging and manufacturing costs. In some conventional EGC valves, in order to open such valves from their closed position, a relatively large amount of energy from the actuator is required. For example, a butterfly valve with a radial seal may be subject to sticking in its closed position in the valve seat. As such, a relatively large amount of energy is required to overcome such sticking. Furthermore, the required energy and force required for opening the valve may be exacerbated by temperature fluctuations in the exhaust system. Soot and condensate build-up may also increase the required break-away force. Some conventional systems have attempted to address this problem, but have not provided a valve design that also provides for accurate flow control, high flow rate capacity and/or exhaust system environment compatibility.
Another consideration for EGC valve design concerns leakage when the valve is in the closed position. It is sometimes desirable for the EGC valve to have a leak proof seal when in a fully closed position. Several attempts have been made to achieve a leak proof seal using butterfly valves and radial seals. There are various designs for fitting a resiliently loaded seal ring into the valve flow channel such that when the valve is in the closed position, the outer periphery of the valve armature pushes against the seal ring. Alternatively, the resilient seal ring may be positioned directly on the outer periphery of the valve armature so that a radial seal is made when the valve closes against the flow channel wall.
One radial seal design provides a relatively leak proof seal, but is relatively expensive and complex. Increased complexity may compromise reliability. In an attempt to minimize complexity, another design includes a formed hard seat positioned in the flow channel. The hard seat mates with a contoured profile on the outer periphery of the valve armature. The valve seat may be machined directly in the valve housing. Alternatively, he valve seat may be machined on a seal ring, and then the seal ring mounted in the housing. Although complexity is slightly decreased, precision machining is required to achieve mating components that seal around the entire circumference of the valve. Manufacturing and assembly costs are again relatively high. Furthermore, most radial seal designs are prone to sticking in the valve seat when subjected to engine temperature fluctuations and debris build-up.
Other designs provide for rotating the armature into a partially seated position and then translating linearly or axially into the seat. A cam or pivot mechanism may be integrated into the valve actuator shaft that engages near its final rotation. This may overcome some sticking problems associated with radial sealing of butterfly valves. However, complexity and cost are again increased. Furthermore, such translating seal designs often fail to provide accurate flow rate control.
Other designs offset the actuator shaft centerline from the valve armature centerline to improve sealing in a butterfly valve. The offset is such that the moment force on the flow side for closing the valve is increased. On the flow side for opening the valve, less area on the armature is exposed. In this way, the force required for opening the valve is decreased. The net moment force is greater on the flow side for closing the valve, thereby enhancing sealing. However, such designs are often subject to sticking, or require expensive precision machining for orienting the sealing surfaces.
Other designs provide for valve designs with flexible face seal contact. Generally, face-sealing valves are not subject to sticking compared to radial seal designs, because a flat surface on the valve armature engages a flat surface on the valve seat. The valve armatures in such designs incorporate discs that are pivotally mounted on the actuator arm. This may allow for self-alignment of the disc on the valve seat when in the closed position. Although precision machining may not be required to align the sealing surfaces of such valves, they fail to provide for fine resolution of flow rate control.
While prior designs have addressed some of the issues and problems associated with EGC valve function and performance, they do not adequately address all of these issues in one EGC valve. Therefore, there is a need for an EGC valve that provides accurate flow control resolution, sufficient flow capacity, the ability to operate reliably in the harsh environment of an exhaust system, leak proof sealing and resistance to sticking. In addition, there is a need for an EGC valve having a failsafe operation that limits or shuts off flow in the event of an actuator failure. Simplicity in design and ease of manufacture is typically important for any EGC valve, given the cost sensitive needs of the automotive industry.